Oklahoma CDL Combination key terms

A power unit towing one or more trailers or semitrailers, joined so they steer and brake as a single unit. In Oklahoma CDL testing it means a tractor pulling a semitrailer, requiring a Class A license.

The front power unit of a combination vehicle, with a cab, engine, rear drive axles, and a coupling device mounted over the drive tandems. It carries part of the trailer's weight through the fifth wheel.

A trailer whose front end rests on and is supported by the towing tractor rather than its own front axle. It attaches through a kingpin and fifth wheel and carries cargo weight on both its own axles and the tractor.

The horseshoe-shaped coupling plate mounted on the tractor's frame that locks around the trailer kingpin. Its jaws must close fully around the pin, and the locking lever must be secured, before the combination is safe to move.

The vertical steel pin under the trailer's front plate that the fifth wheel jaws grip. It carries the pulling and braking forces between tractor and trailer, so a worn, bent, or improperly seated kingpin can let the trailer separate.

The flat plate on the underside of a semitrailer's front end that rides on the fifth wheel. It must sit flush on the fifth wheel with no gap; a visible gap means the kingpin is not locked in.

The procedure of backing the tractor under a semitrailer and locking the fifth wheel to the kingpin, then connecting air lines and electrical cord. Done wrong, it lets the trailer drop or separate on the road.

The procedure of parking a combination, lowering the landing gear, disconnecting air and electrical lines, releasing the fifth wheel, and pulling the tractor clear. Skipping steps can drop the trailer or damage lines and gear.

After coupling, pulling the tractor gently forward against the locked trailer with the trailer brakes set, then checking that the trailer did not move and the gap stayed closed. It confirms the fifth wheel jaws actually grabbed the kingpin.

The retractable legs under a semitrailer's front that support it when uncoupled from a tractor. They must be fully raised before moving, and lowered until firm ground contact before the tractor is pulled out.

The two flexible hoses, service (usually blue) and emergency/supply (usually red), that carry air between tractor and trailer. They must be connected, sealed, and free of chafing, or trailer brakes will not release or apply correctly.

The interlocking couplers on the ends of the tractor and trailer air hoses. They must be locked together with their sealing gaskets intact and the supply line on the tractor matched to the trailer's supply coupler.

A braking system that uses compressed air to apply the service brakes and spring brakes. In a combination, air must charge the trailer tanks, so pressure buildup and leak-down times matter for the pre-trip inspection.

Parking and emergency brakes held applied by heavy springs and released by air pressure. If air pressure drops too low, they apply automatically, which is why a combination with a major leak stops rather than rolls freely.

A valve that closes off the tractor's air supply if a trailer breaks away or a line fails, keeping enough air to apply the tractor's brakes. It also lets the trailer's spring brakes set when pressure is lost.

A pre-trip check where the system is charged, the engine shut off, and pressure loss measured over one minute with brakes released and then applied. Excessive loss means leaks that must be fixed before driving.

An electronic system that rapidly releases and reapplies brake pressure at individual wheels to prevent lockup during hard braking. It helps retain steering control and reduces wheel-hop and trailer swing, but does not shorten stopping distance on all surfaces.

The yellow ABS light on the dash or trailer that signals a fault in the antilock system. A lamp that stays on after the self-test means ABS may not work, though normal brakes remain, and the fault should be reported.

A component at each wheel that reports rotation speed to the ABS controller. A damaged sensor or tone ring can switch off antilock function for that wheel and trigger the warning lamp.

The systematic walk-around and in-cab check of a combination before driving, covering coupling, air brakes, lights, tires, cargo securement, and load limits. It is a scored part of the Oklahoma CDL skills test.

The tendency of a trailer's rear wheels to follow a path inside the tractor's front wheels in a turn. The longer the combination and the sharper the turn, the more the trailer cuts the corner, so drivers swing wide and watch mirrors.

The hazard of a right turn where the trailer's rear wheels track inside the tractor's path, threatening to crush a pedestrian, cyclist, or object at the curb. Drivers must check mirrors and avoid drifting left before turning.

The total width of road a combination occupies as it moves through a turn, including the tractor's front swing and the trailer's inside cut. Planning for the swept path prevents curb strikes and lane encroachment.

The size of the arc a combination needs to complete a turn. Longer wheelbases and trailers increase it, so a driver may need to begin the turn later, use more of the lane, or take a wider approach.

Keeping the tractor and trailer centered and within lane lines, using mirrors and reference points rather than guesswork. Poor lane control in a combination causes trailer sway, sideswipes, and lane-departure crashes.

The frequent use of both flat and convex mirrors to monitor trailer position, blind spots, and traffic alongside. In a combination, mirrors are the main way to confirm the trailer is tracking where intended.

An area around the combination that mirrors do not show, including directly behind the trailer, low alongside the cab, and the right side. Drivers must turn the head and use mirrors before lane changes and backing.

Moving a combination in reverse, which reverses the trailer's response: turn the wheel left and the trailer's rear goes right. Back slowly, use a spotter when possible, and get out to look if unsure.

A person outside the vehicle who guides a backing maneuver with agreed hand signals. The driver stops immediately if the spotter is out of view, since the spotter covers areas mirrors cannot show.

A loss of control where the tractor and trailer fold against each other, either from locked drive wheels swinging the tractor or from a trailer skidding sideways. It often results from hard braking on slick or empty-brake surfaces.

A skid in which the trailer's rear wheels lose traction and slide sideways, swinging the trailer out of line. It is corrected by releasing the brakes and steering in the direction the trailer is sliding.

Loss of tire traction where wheels slide instead of roll, removing steering and braking control. Recovery means releasing hard braking, looking and steering where you want to go, and avoiding abrupt inputs.

The margin of clear space kept around a combination so there is time to react to other drivers' mistakes. It grows with speed and weight, and following distance should increase with load and road conditions.

The gap kept behind the vehicle ahead, measured in seconds by counting from when it passes a fixed point. A loaded combination needs at least one second per ten feet of vehicle length plus extra for speed and weather.

The skill of scanning ahead and recognizing developing risks, such as merging traffic, work zones, or stopped vehicles, early enough to act smoothly. Looking far ahead and checking mirrors regularly supports it.

Choosing a speed that matches road, weather, visibility, load, and braking limits, and slowing before curves, ramps, and downgrades. A heavy combination needs far more distance to stop, so speed must be managed early.

Tying down and containing cargo so it cannot shift, spill, or fall, using approved tiedowns, straps, chains, and blocking. Federal rules require a minimum number of tiedowns and working load limits based on cargo weight.

The maximum load a tiedown, strap, chain, or fitting is rated to hold, marked by the manufacturer. The combined WLL of the tiedowns must at least equal half the cargo weight under the general securement rule.

Placing cargo so weight is spread correctly across steering, drive, and trailer axles without overloading any one of them. Poor distribution can lighten the steering axle, overload tires, or exceed bridge and axle limits.

The maximum weight allowed on a single axle or axle group, set by federal and Oklahoma rules and by tire ratings. Loads must be positioned so no axle or tire exceeds its limit.

The maximum total weight a vehicle or combination is designed to carry, set by the manufacturer. Exceeding it, or the registered weight, can create unsafe handling and legal violations.

Movement of cargo during driving, which can change axle loads, damage the trailer, or spill onto the road. Proper tiedowns, blocking, and load placement prevent it.